What is the difference between a TT and an ETT? And what makes the PS decide which procedure to perform?

Good question. It Simply has to do with the amount of skin that is removed on the sides of the abdomen, closer to the hips. The reason for this is that some patients with less skin can get away with just a standard tummy tuck incision with muscle tightening. While others with more redundant or excessive abdominal skin will require additional removal. It is just a different term for tummy tuck with a longer incision. However, this also means that an extended tummy tuck would cost more than a standard tummy tuck due to the additional time required during surgery.

It simply has to do with the amount of skin laxity the patient has, and the tailoring needed to correct it.  Someone with more laxity, especially towards the sides of the abdomen, is more likely to need an extended approach.